This weakaura is to help Feral Druids to know when to clip (ie. refresh it before it expires) their Savage Roar.
You can set both the offset and the leeway in the Custom Options. By default, the offset is automatic (if you check "Automatic Offset" and its value is 31 for 4P T8 and 25 otherwhise. For the leeway, by default its value is 4. These values are optimal according to the latest simulations (P3 SIM).
Credits to NerdEgghead for the maths.